1. Field of the Invention
The present invention generally relates to a method and apparatus for inspecting objects and more particularly to a method and apparatus for performing film surface detection (e.g., oxide film etching endpoint detection, film surface contamination detection, etc.) for overlayers on a sample (e.g., a silicon wafer).
2. Description of the Related Art
Fourier-Transform infrared (FTIR) spectroscopy can be used as an extremely sensitive endpoint detection scheme for the etching of thin film overlayers on silicon (e.g., silicon dioxide thin film etching by hydrogen fluoride vapor (HF)). This operation is performed by monitoring the appearance of vibrational features, such as those associated with the hydrogen-passivated silicon surface. In recent tests, a native oxide layer has been removed from the silicon surface by vapor phase HF etching prior to the deposition of tungsten silicide. To obtain an adequate real-time, signal-to-noise ratio, a sample geometry allowing multiple reflections is necessary in which infrared (IR) radiation interacts with the silicon surface a plurality of times. Indeed, for real-time measurement a single pass of the beam through the surface will not be adequate due to the poor signal-to-noise ratio.
Conventionally, the multiple reflections of the IR radiation have been achieved only by using specially prepared or modified silicon substrates in which the infrared radiation is made to propagate within the sample (as compared to propagating externally through the sample). The specially prepared or modified silicon substrates may include bevelled ends which are typically cut at a 45-degree angle at each end, to ensure that the IR radiation beam propagates internally and interacts with the surface a plurality of times before exiting from the other end. The specially prepared or modified silicon substrates are costly and inefficient to produce. Indeed, :he specially prepared wafers which are bevelled or cut at a 45-degree angle at each end result in the wafer being typically broken or destroyed and thus the wafer is rendered worthless in terms of further processing. Further, the use of monitor wafers would not be indicative of the surface condition of the product wafers such as the presence of contamination or species left on the surface. These attributes vary from wafer-to-wafer and from lot-to-lot.
Hence, for the foregoing reasons, the geometry of the specially prepared silicon substrates is not transferable to conventional silicon wafers (i.e.. absent the special preparations thereof) and indeed precludes using the FTIR spectroscopy as a real-time surface monitoring method for conventional silicon wafers since such wafers are not, as a practical matter, built to have bevelled ends or the like. Further, to build such wafers or modify conventional wafers to have the special geometry is inefficient and adds extra processing steps.
Thus, the conventional arrangement does not support multiple interactions with the surface of conventional silicon wafers, let alone multiple external interactions with the surface. Indeed, such arrangements (e.g., wafers having bevelled or cut ends) do not support external transmission and reflection through the wafer.
Therefore, a problem of the conventional systems is that there is no apparatus and method for performing real-time, nondestructive, in situ measurements of surface properties of a conventional wafer using multiple external transmission and reflection.